Dissolved zinc and zinc-complexing ligands in an organic-rich estuary: Benthic fluxes and comparison with copper speciation

We report the first comprehensive set of direct measurements of benthic fluxes of zinc-complexing ligands from estuarine sediments, using intact cores in controlled incubation experiments. Water column concentrations of total dissolved Zn in the Cape Fear River estuary (southeastern North Carolina) ranged from ≤ 5–47 nM in the upper estuary and ≤ 5–64 nM in the lower estuary compared to concentrations of ligands (log K′Zn−ΣL,Zn2+), of 10–102 and 5–67 nM, respectively. Benthic fluxes of total dissolved Zn (− 1100 to 1800 nmol m− 2 d− 1) and ligands (− 1700 to 2900 nmol m− 2 d− 1), where positive fluxes represent a movement out of the sediment, were sporadic and did not contribute significantly (< 6%) to the standing stocks of Zn and ligands in the estuary. Effluxing ligands were indistinguishable from those found in the water column, indicating that sediments are a potential source of Zn ligands to estuarine waters, particularly in systems with longer water residence times and reactive, fine-grained sediments. Titrations of solutions of C18-extractable humic substances extracted from the Cape Fear River indicated that they are not strong complexants of Zn, in contrast to previous work showing that humics dominate strong Cu complexation in the estuary. Comparisons of Zn speciation in this study to previous results on Cu speciation in the estuary indicate that benthic fluxes and water column speciation of both metals are decoupled from one another.